New dyslexia theory blames 'noise'

Poor filtering of unwanted data may be the root cause of common disorder

The dyslexic brain struggles to read because even small
distractions can throw it off, according to a new model of
dyslexia emerging from a group of recent studies.

The studies contradict an influential, 30-year-old theory that
blamed dyslexia on a neural deficit in processing the fast
sounds of language.

Instead, the studies suggest that children with dyslexia
have bad filters for irrelevant data. As a result, they struggle
to form solid mental categories for identifying letters and
word sounds.

Such children may benefit from intensive training under
"noisy" conditions to strengthen their mental templates,
said University of Southern California neuroscientist
Zhong-Lin Lu.

Lu was a co-author on three studies, along with lead author
and former USC graduate student Anne Sperling (now at
the National Institute of Mental Health), USC psychologist
Franklin Manis and University of Wisconsin, Madison
psychologist Mark Seidenberg.

The most recent study is due to be published later this
month in Psychological Science.

Confusion about dyslexia rivals the confusion of dyslexia.
Many still think that to have dyslexia means to mix up your
letters (one of many possible symptoms having to do with
word recognition, directional ability and decoding of
symbols).

What is known is that dyslexia affects millions of children,
with estimates of its incidence ranging from 5 to 15
percent.

Sperling, who conducted her research as a doctoral
student at USC, said the new findings point to a deeper
problem - not just a visual deficit - affecting all areas of
perception.

Sperling said people with dyslexia appear to have shaky
mental categories for the essential sounds that make up
language.

"It's harder to make a [language] task automatic when your
categories are fuzzier than they ought to be to begin with,"
she said.

"In terms of treatment, the results suggest that programs
that foster the development of sharper perceptual
categories for letters and letter sounds might be a good
way to supplement existing dyslexia interventions," she
added.

Lu said, "Train them in noise."

The new study in Psychological Science builds on similar
results published by the team of Sperling, Lu, Manis and
Seidenberg last year in Nature Neuroscience.

In addition, the same authors previously showed that poor
readers also have trouble figuring out categories in simple
card games.

Other recent studies lend support to the noise exclusion
theory.

Johannes Ziegler of the Universite de Provence in
Marseille, France, was the lead author on a study of
dyslexia and auditory noise published this year in
Proceedings of the National Academy of Sciences.

Ziegler said his results suggest that dyslexia stems from
shaky categories for phonemes (the basic sounds of
language).

"In silence, information is often redundant and dyslexics get
away with the perception deficit," Ziegler said in an e-mail.
"In noise, however, they can no longer compensate.

"What is important is that noisy environments are the rule
and not the exception," he added, citing a study from South
Bank University in the U.K. that found average noise levels
in primary classrooms to be as high as near a busy
intersection.

"What Sperling and Lu's data suggest is that the
mechanism responsible for faulty phonological
development is quite general and has to do with attention
in a broad sense....

"This is a great paper of very high significance... As people
like Steve Grossberg [of Boston University] have argued for
many years, attention ... is crucial for stable learning of
categories."

Ziegler called for preventive training for children with weak
speech perception in noise in kindergarten or early primary
grades, saying they are at greater risk for developing
dyslexia.

He also cited a Northwestern University study from 2003
that documented negative effects from noise on children
with learning deficits.

Lu said there is a "lot of evidence" of learning problems
from ambient noise. In one such study, Manis and a
collaborator from UCLA found that children with dyslexia
struggled to discriminate similar sounds, like "spy" and
"sky," because they weighed irrelevant differences in
sounds equally with key distinctions.

Manis also cited research from Finland and the United
States showing that infants with dyslexic parents lag
behind their peers in forming categories for speech
sounds.

In the conclusion to their study in Psychological Science,
the authors speculate that the deficit in noise exclusion
may have biochemical roots in abnormal levels of GABA, a
neurotransmitter that helps the brain to filter out irrelevant
information.

"This may become interesting for drug development," said
Lu, who is testing this hypothesis with functional magnetic
resonance imaging trials.

Lu and his collaborators interpret the new results as a
rejection of the "magnocellular hypothesis" - named for a
type of neuron involved in processing fast visual
information - that influenced dyslexia research for
decades.

The researchers found that the magnocellular pathway
works normally both in children with dyslexia and in adult
poor readers - as long as visual or aural noise is low.

As external noise goes up, the same subjects begin to
score poorly on visual pattern tests.

The deficit persists even when the task requires only slow
processing.

"The findings, and particularly the [slow processing] ones,
are consistent with the hypothesis that ... dyslexic children
have difficulty setting their signal filters to optimum and
ignoring distracting noise," Lu said at the time of the Nature
Neuroscience study.

The new study in Psychological Science was designed to
replicate visual tests on motion perception from seminal
experiments in the 1970s, with the addition of variable
external noise. It also found no magnocellular deficit.

"These were the stimuli people used to establish the
magnocellular hypothesis," Lu said. "This is a more direct
test of what we said before [in the Nature Neuroscience
study], which used different spatial and temporal patterns."

###

Funding for this research came from a grant to Manis from
the National Institute of Child Health and Human
Development.

The NICHD defines dyslexia as "a specific learning
disability that is neurobiological in origin. It is characterized
by difficulties with accurate and/or fluent word recognition
and by poor spelling and decoding abilities. These
difficulties typically result from a deficit in the phonological
component of language that is often unexpected in relation
to other cognitive abilities and the provision of effective
classroom instruction.

Secondary consequences may include problems in
reading comprehension and reduced reading experience
that can impede growth of vocabulary and background
knowledge."

Last reviewed:
By John M. Grohol, Psy.D. on
30 Apr 2016
Published on PsychCentral.com. All rights reserved.